Chemical vapor deposition (CVD) is a process of forming a film on a substrate, typically, by generating vapors from liquid or solid precursors and delivering those vapors to the surface of a heated substrate where the vapors react to form a film. Systems for chemical vapor deposition are employed in applications such as semiconductor fabrication, where CVD is employed to form thin films of semiconductors, dielectrics and metal layers. Three types of systems commonly used for performing CVD include bubbler-based systems, liquid-mass-flow-control systems, and direct-liquid-injection systems.
Bubbler-based systems, or "bubblers," essentially bubble a stream of gas through a heated volume of liquid precursor. As the stream of gas passes through the liquid precursor, vapors from the liquid precursor are absorbed into the gas stream. This mixture of gases is delivered to a process chamber, where the gases react upon a surface of a heated substrate. Bubblers typically heat the volume of liquid precursor at a constant temperature. Over time, the constant heat often causes the precursor to decompose rendering it useless for CVD. In an effort to minimize decomposition, the bubbler is typically maintained at a temperature lower than that at which the vapor pressure of the liquid precursor is optimal.
Liquid mass flow control systems attempt to deliver the precursor in its liquid phase to a vaporizer typically positioned near the substrate. The precursor is vaporized and is then typically entrained in a carrier gas which delivers it to the heated substrate. A liquid mass flow controller, which is a thermal mass flow controller adapted to control liquids, is used to measure and control the rate of flow of liquid precursor to the vaporizer.
Liquid mass flow controllers present a number of drawbacks. First, liquid mass flow controllers are extremely sensitive to particles and dissolved gases in the liquid precursor. Second, liquid mass flow controllers are also sensitive to variations in the temperature of the liquid precursor. Third, liquid mass flow controllers typically use a gas to assist in the vaporization of the liquid precursor, thereby increasing the probability of generating solid particles and aerosols and ensuring a high gas load in the process system. Fourth, most liquid mass flow controllers cannot operate at temperatures above 40.degree. C., a temperature below which some precursor liquids, such as tantalum pentaethoxide (TAETO), have high viscosity. Due to its sensitivities, the liquid flow controller is accurate and repeatable to about 1% of full-scale liquid flow. Further, when a liquid mass flow controller wetted with TAETO or one of a number of other precursors is exposed to air, the precursor will generally react to produce a solid which may destroy the liquid flow controller.
Liquid pump-based systems pump the liquid precursor to the point of vaporization, typically at a position near the heated substrate. Liquid pump-based systems are generally one of two main types. One type uses a liquid flow meter in line with a high-pressure liquid pump. The other type uses a high-precision, high-pressure metering pump. Both of these systems are extremely sensitive to particles in the liquid. The liquid-flow-meter based system is also sensitive to gas dissolved in the liquid. Both are extremely complex to implement, and neither can tolerate high temperatures (maximum 50.degree. C.). The system with the metering pump has difficulty vaporizing high viscosity liquids. Finally, both are generally difficult to implement in a manufacturing environment due to their extreme complexity and large size.
Existing CVD equipment design is generally optimized for high process pressures. The use of high process pressures is most likely due to the fact that, until recently, CVD precursors were either generally relatively high-vapor-pressure materials at room temperature or were, in fact, pressurized gases. Examples include tetraethyloxy silicate (TEOS), TiCl.sub.4, Silane, and tungsten hexafluoride, etc. These materials were chosen because they had high vapor pressures and could therefore be easily delivered. The process pressure was generally well within the stable vapor pressure range of each of these materials.